diff --git a/app/scripts/platforms/extension.js b/app/scripts/platforms/extension.js
index 6f30157fe..0626f1545 100644
--- a/app/scripts/platforms/extension.js
+++ b/app/scripts/platforms/extension.js
@@ -92,14 +92,16 @@ export default class ExtensionPlatform {
       // On Chrome, a more descriptive representation of the version is stored
       // in the `version_name` field for display purposes.
       return versionName;
-    } else if (versionParts.length === 4) {
+    } else if (versionParts.length !== 3) {
+      throw new Error(`Invalid version: ${version}`);
+    } else if (versionParts[2].match(/[^\d]/u)) {
       // On Firefox, the build type and build version are in the fourth part of the version.
-      const [major, minor, patch, prerelease] = versionParts;
-      const matches = prerelease.match(/^(\w+)(\d)+$/u);
+      const [major, minor, patchAndPrerelease] = versionParts;
+      const matches = patchAndPrerelease.match(/^(\d+)([A-Za-z]+)(\d)+$/u);
       if (matches === null) {
         throw new Error(`Version contains invalid prerelease: ${version}`);
       }
-      const [, buildType, buildVersion] = matches;
+      const [, patch, buildType, buildVersion] = matches;
       return `${major}.${minor}.${patch}-${buildType}.${buildVersion}`;
     }
 
diff --git a/app/scripts/platforms/extension.test.js b/app/scripts/platforms/extension.test.js
index c04d0b0f4..af7852b8b 100644
--- a/app/scripts/platforms/extension.test.js
+++ b/app/scripts/platforms/extension.test.js
@@ -39,7 +39,7 @@ describe('extension platform', () => {
 
     it('should return SemVer-formatted version for Firefox style manifest of prerelease', () => {
       extension.runtime.getManifest.mockReturnValue({
-        version: '1.2.3.beta0',
+        version: '1.2.3beta0',
       });
       const extensionPlatform = new ExtensionPlatform();
 
@@ -66,14 +66,21 @@ describe('extension platform', () => {
       });
       const extensionPlatform = new ExtensionPlatform();
 
-      expect(() => extensionPlatform.getVersion()).toThrow(
-        'Version contains invalid prerelease:',
-      );
+      expect(() => extensionPlatform.getVersion()).toThrow('Invalid version:');
+    });
+
+    it('should throw error if version includes four parts in a Firefox style manifest', () => {
+      extension.runtime.getManifest.mockReturnValue({
+        version: '1.2.3.4',
+      });
+      const extensionPlatform = new ExtensionPlatform();
+
+      expect(() => extensionPlatform.getVersion()).toThrow('Invalid version:');
     });
 
     it('should throw error if build version is missing from Firefox style prerelease manifest', () => {
       extension.runtime.getManifest.mockReturnValue({
-        version: '1.2.3.beta',
+        version: '1.2.3beta',
       });
       const extensionPlatform = new ExtensionPlatform();
 
@@ -82,9 +89,9 @@ describe('extension platform', () => {
       );
     });
 
-    it('should throw error if build type is missing from Firefox style prerelease manifest', () => {
+    it('should throw error if patch is missing from Firefox style prerelease manifest', () => {
       extension.runtime.getManifest.mockReturnValue({
-        version: '1.2.3.0',
+        version: '1.2.beta0',
       });
       const extensionPlatform = new ExtensionPlatform();
 
diff --git a/development/build/utils.js b/development/build/utils.js
index ad125dcad..59e4fd7c3 100644
--- a/development/build/utils.js
+++ b/development/build/utils.js
@@ -50,7 +50,7 @@ function getBrowserVersionMap(platforms) {
     const browserSpecificVersion = {};
     if (prerelease) {
       if (platform === 'firefox') {
-        versionParts.push(`${buildType}${buildVersion}`);
+        versionParts[2] = `${versionParts[2]}${buildType}${buildVersion}`;
       } else {
         versionParts.push(buildVersion);
         browserSpecificVersion.version_name = version;